This invention relates to a double-walled vacuum insulated container for product storage at cryogenic temperatures.
Double-walled vacuum insulated containers are widely used for the long-term preservation of living tissue, sperm and whole blood and for storage and tranportation of valuable cryogenic liquids. These containers usually employ in the vacuum insulation space, a composite multi-layered, external load-free insulation comprising low conductive fibrous sheet material layers composed of fibers for minimizing heat transfer by solid conduction, and thin flexible sheet radiation barrier layers. The radiation barrier layers are supportably carried in superimposed relation by the fibrous sheet layers to provide a large number of radiation barrier layers in a limited space for reducing the transmission of radiant heat across the vacuum space without perceptively increasing the heat transmission by solid conduction thereacross. Each radiation barrier layer is disposed in contiguous relation on opposite sides with a layer of the fibrous sheet material, the fibers being oriented substantially parallel to the radiation barrier layers and substanially perpendicular to the direction of heat inleak across the insulating space.
One such commonly used fibrous sheet material described in Matsch U.S. Pat. No. 3,009,600 is small diameter glass fibers (about 0.5 microns diameter) in permanently precompacted sheets of about 1.5 mils thick and weighing about 1.5 gms. per sq. ft. (hereinafter referred to as "glass microfiber"). The extreme finess of glass microfiber affords mechnical integrity of the separator in very thin sheet form without reliance on chemical binders to "glue" the fibers together. When an insulation comprising glass microfiber sheets alternating with thin aluminum foils is installed at near-optimum density of 70 layers/inch and in a vacuum of less than 0.1 micron Hg absolute, its thermal conductivity is about 2.5 .times.10.sup..sup.-5 Btu/hr.sup.. ft.sup.. .degree. F. If a 29-liter liquid nitrogen container is provided with such an insulation, it is capable of obtaining a normal evaporation rate (NER) of about 0.33 lbs. of the liquid nitrogen per day.
The disadvantages of glass microfiber are its high cost and its extreme sensitivity to mechanical compression. The latter characteristic has been explained as the result of increasing the number of fiber-to-fiber contacts within the sheet which in effect shortens the path of heat flow between the reflective foils separated by the sheet. In practical usage of thin permanently precompacted-form spacers in multiple layer insulation, it is usually impossible to avoid high compression at least in localized areas of the insulation.
An alternative glass fiber material, described in Clapsadle U.S. Pat. No. 3,145,515, is large diameter (1.6-2.6 micron) fibers in fluffy uncompacted "web" sheets without significant binder. Lack of strength and poor handleability, characteristics of this separator, are accommodated by supporting the fiber sheet continuously on another, stronger sheet material such as the reflective foil used in the insulation. Thus, the supporting foil may be interleaved with the delicate fiber sheet at the time the latter is produced, and thereafter, the two components are handled and applied together during vessel manufacture as a single composite layer. The resultant multi-layer insulation is excellent for large vessels requiring moderately effective insulation, but its thermal conductivity (about 10 .times. 10.sup..sup.-5 Btu/hr.sup.. ft.sup.. .degree. F) does not meet the requirements for small cryogenic containers with long "holding" time.
An alternative to glass fiber sheets are the organic fiber separators described in Gibbon et al U.S. Pat. No. 3,265,236 having certain specifications including much lower intrinsic thermal conductivity than glass. By way of example, the patent states that with a rayon fiber, a minimum thermal conductivity for multi-layer insulation is obtained which is equal to glass fiber multi-layer insulation, but with fiber 16 to 24 times larger in diameter. In order to obtain strength and good handling characteristics with large fibers in thin sheets, the patent contemplates the use of binders such as polyvinylacetate in quantity such as 14 wt. % of the sheet. Sheet materials weighing 1.475 and 1.01 gms/ft..sup.2 are disclosed. In addition to rayon, other disclosed suitable organic fiber materials are cotton, Dacron, Dynel and nylon. Dacron is a polyester produced by condensation of dimethylterephthalate, nylon is a polyamide and Dynel is a copolymer of vinyl chloride-acrylonitrile.
According to the Gibbon et al patent, fiber sheets may be produced from these organic materials using either paper-making or textile machinery. Textile sheets have not been used in commercial installations, however, due to relatively high cost and poor thermal efficiency. In paper-making machinery, the fibers are laid down on a moving screen and are compressed while wet as between rolls, so that after drying, the paper retains a compressed condition. Sheet materials produced of large diameter rayon fibers (e.g. 12-18 microns), in low thicknesses (e.g. 1-2 mils) and in light weight (e.g. 0.8 to 1.5 gms/ft..sup.2) afford satisfactory separators for composite insulations. One such material applied at near-optimum layer density of about 70 layers per inch provides a thermal conductivity on the order of 2 .times. 10.sup..sup.-5 Btu/hr.sup.. ft.sup.. .degree. F. The fiber sheets are reasonable in cost, being readily produced on wet-process, paper-making machinery.
The NER value for the above-described rayon fiber-aluminum foil multi-layered container is about 0.272 lbs/day of nitrogen based on a minimum 40 hour pump down time and an average (cold) vacuum pressure of about 0.13 micron Hg. It would be desirable to provide a container having a multi-layered insulation with even better thermal performance, i.e., lower thermal conductivity which permits lower NER. By way of example, for a 29 liter capacity container, the "holding time" for storing products based on the aforementioned NER is about 190 days. If this NER could be reduced by 8% to about 0.251 lbs/day, the holding time may be increased to 206 days before the container needs to be refilled with liquid nitrogen.
One object of this invention is to provide an improved multi-layered thermal insulation system for the vacuum space of double-walled cryogenic storage containers, characterized by low heat conductance and low material costs.
Other objects will be apparent from the ensuing disclosure and appended claims.